In recent times, more and more interest has focused on flame retardant materials for the construction of residential, commercial and industrial buildings. Consequently, great emphasis is being placed on compositions which are capable of imparting the desired flame retardancy to building materials, such as cellulosic boards, without, however, deteriorating the essential physical properties of the final products.
Within the last few years, several flame retarding compositions have been suggested for cellulosic boards. Thus, in U.S. Pat. No. 4,008,214 Feb. 15, 1977), hard and particleboards have been rendered flame retardant by incorporating in these boards a synergistically acting composition containing alumina hydrate in combination with a melamine-formaldehyde phosphate, urea-formaldehyde phosphate or dicyanamide-formaldehyde phosphate.
In German Application (Offenlegungsschrift) No. 2,627,682 -- Moore et al, published on Jan. 20, 1977, cellulosic products, such as hardboards, are rendered flame retardant by incorporation in the boards a composition consisting of alumina hydrate and a boron source. As boron source B.sub.2 O.sub.3, H.sub.3 BO.sub.3 and ammonium borates are described. In this published application, reference is also made to compositions containing mixtures of alumina hydrate with monoammonium phosphate or diammonium phosphate. These compositions have been found to be inferior in their flame-retarding effects for cellulosic boards.
In U.S. Pat. No. 3,865,760 (Feb. 11, 1975) -- Pitts et al, rubber or plastic compositions are rendered flame retardant by incorporation of an agent consisting of a naturally occurring boron-containing mineral, such as colemanite, either alone or in combination with alumina hydrate.
In our co-pending U.S. application, Ser. No. 780,725, filed Mar. 24, 1977, now U.S. Pat. No. 4,076,580, a synergistically acting ulexite-alumina hydrate flame-retarding composition is described, being capable of imparting Class I fire rating to cellulosic boards. By careful control of the loading of this composition, a flame-retarded product is obtained which conforms to Class I fire rating, while exhibiting suitable mechanical properties.
It has now been discovered that significantly improved flame retardancy can be imparted to cellulosic boards, while simultaneously retaining the important physical properties of these boards, by using a three-component flame-retarding agent consisting of a source of Al.sub.2 O.sub.3.xH.sub.2 O, a naturally occurring boron-containing mineral, and an inorganic phosphate or sulfate containing cosynergist, to be defined in detail hereafter.
The instant flame-retarding composition consistently provides Class I fire ratings or better, without generation of undesirable afterglow, by the unexpected interaction of the components.